26 CFR § 48.4161(a)-3 - Parts and accessories.
(a) In general. The tax attaches with respect to parts and accessories for articles specified in section 4161(a) and § 48.4161(a)–1 that are sold on or in connection with such articles, or with the sale thereof, at the same rate applicable to the sale of the basic articles. The tax attaches in such cases whether or not charges for the parts or accessories are billed separately. To be considered a part or accessory for an article specified in section 4161(a), an item must be either essential to the operation of the specified article, or be designed to directly improve the performance of the specified article, or to improve its appearance. For example, a carrying case for a fishing rod is not considered to be a part or accessory for a fishing rod, despite the fact that it is designed for use with the rod, because it is neither essential to the use of the rod, nor does it in any way improve its performance or appearance. A sale of a part or accessory which would otherwise be considered a sale “on or in connection with” the sale of an article taxable under section 4161(a), is not subject to tax if the part or accessory is sold as a replacement for an identical part or accessory being sold with the taxable article.
(b) Essential equipment. If taxable articles are sold by the manufacturer, producer, or importer thereof, without parts or accessories that are essential for their operation, or are designed directly to improve the performance or appearnace of the articles, the separate sale of the parts accessories to the same vendee will be considered, in the absense of evidence to the contrary, to have been made in connection with the sale of the basic article, even though the parts or accessories are shipped separately at the same time or on a different date.